# usage: build_index(*DATA_HANDLE, *INDEX_HANDLE)
sub build_index {
    my $data_file  = shift;
    my $index_file = shift;
    my $offset     = 0;
 
    while (<$data_file>) {
        print $index_file pack("N", $offset);
        $offset = tell($data_file);
    }
}
 
# usage: line_with_index(*DATA_HANDLE, *INDEX_HANDLE, $LINE_NUMBER)
# returns line or undef if LINE_NUMBER was out of range
sub line_with_index {
    my $data_file   = shift;
    my $index_file  = shift;
    my $line_number = shift;
 
    my $size;               # size of an index entry
    my $i_offset;           # offset into the index of the entry
    my $entry;              # index entry
    my $d_offset;           # offset into the data file
 
    $size = length(pack("N", 0));
    $i_offset = $size * ($line_number-1);
    seek($index_file, $i_offset, 0) or return;
    read($index_file, $entry, $size);
    $d_offset = unpack("N", $entry);
    seek($data_file, $d_offset, 0);
    return scalar(<$data_file>);
}
 
# usage:
$file = 'E:/test/lin_common_proto.c';
open(FILE, "< $file")         or die "Can't open $file for reading: $!\n";
open(INDEX, "+>$file.idx")
        or die "Can't open $file.idx for read/write: $!\n";
build_index(*FILE, *INDEX);
$line = line_with_index(*FILE, *INDEX, $seeking);
print "LINES: \n" . $line;